Technical Abstract:
Soil is an important source and sink of active carbon and plays a major role in the global carbon cycling. The increasing concern on greenhouse gases emission and global climate change requires estimates of global soil carbon pool, especially the bioreactive soil organic carbon (SOC) fraction. The current SOC fractionation methods are not designed to differentiate and estimate the magnitude of bioreactive SOC pool. Recent results have shown that the NCSOIL model can reasonably estimate the size of the bioreactive SOC pool in a soil. Experimental studies were designed to reach for an analytical method that could approximate the bioreactive SOC as defined by the NCSOIL method. Studies were conducted to determine the potential of acid hydrolysis for estimating the bioreactive fraction of SOC in soils. Soils were hydrolyzed under reflux with 1 M or 6 M HCl from 1 to up to 24 hours. Results showed that 1.7 to 3.2% of organic C as CO2 could be liberated from the acid hydrolysis of soil, and 28.6 to 34.0% of organic C was solubilized but not mineralized. Higher amounts of soluble organic C and N as well as CO2 were released from soils by 6 M HCl than by 1M HCl. Soluble organic C and N contents in both 1 M HCl and 6 M HCl hydrolysates of soils increased rapidly during the initial 2 hours of hydrolysis, and then increased gradually. The amounts of CO2-C evolved was significantly correlated with the amounts of NH**4-N released at different times during the acid hydrolysis (r=0.88-0.96). The ratio of organic C to soluble N was lower in 6M HCl hydrolysate than in 1 M HCl. Hydrolysis of soil by 1 M HCl for 4 h appeared to be a promising approach for estimating the more bioreactive pools of SOC.